Demand for plastic containers such as bottles is expected to increase in the future. At the same time, many efforts are being made to reduce the thickness of bottles so as to lower their cost and reduce volume at the time of waste disposal.
Generally, plastic bottles are classified into two kinds by manufacturing method: direct blown bottles and draw blown bottles. In the direct blow method, plastic bottles are manufactured by blow molding a hot parison in a die immediately after heating, melting and then extruding the parison through the die head of an extruder (melt blow molding). In the draw blow method, plastic bottles are manufactured by cooling a parison extruded from an extruder or injected from an injector, thereafter heating the cooled parison again, to a temperature for drawing and then blow molding it in a die (stretch blow molding).
Conventionally, the method where a hot parison is blow molded just after extrusion from an extruder has been generally used in view of thermal economy.
In order to reduce the thickness of direct blow bottles, it is, taking blow ratio into account, necessary to extrude a parison as thin as possible from a die head.
In melt blow molding, the space between the core and the shell in a die head (die) is changed to adjust the thickness of the parison locally. Japanese Patent Laid-open No. S62(1987)-202713 teaches a method for making the adjustment by moving the core upward or downward, whereas Japanese Patent Laid-open No. H5(1993)-104525 teaches a method for making the adjustment by changing the position of the shell.
In such a conventional technique of changing the space between the core and shell for adjusting the thickness of the parison, it is easy to gradually change the thickness in small degrees but it is difficult to abruptly change in the thickness by a large degree.
This is because a large taper angle of the core is required for abruptly changing the thickness of the parison in the die head. However, if the taper angle is large, the pressure for extruding the resin and the discharge amount of the resin cannot be sufficiently controlled. As a result, the thickness of the parison cannot be precisely adjusted. These methods are therefore not practical.
The conventional techniques are not capable of reducing the thickness of the body and shoulder while maintaining a specified thickness of the mouth. If the body and shoulder are made thinner, the mouth is disadvantageously made thinner at the same time. Such a thin mouth may deform at the time of capping, leading to insufficient sealing that may cause problems such as liquid leakage. Further, reamer finishing of the diameter and surface of the mouth becomes difficult.
By the conventional techniques, it was difficult to make only a part of the body thick, while maintaining a thin bottle thickness overall. Thus, there has been a limit to the degree that the thickness of a bottle can be reduced and still obtain the desired strength. There has also been a limit to the degree that a bottle can be imparted with decorative property and the degree that its direction of collapse during disposal can be controlled (the amount of functionality that can be imparted).
This invention was accomplished in light of the foregoing circumstances. One object of the invention is therefore to provide a plastic container, such as a bottle, of reduced thickness and enhanced strength, decorative appearance and functionality obtained by making the mouth and/or specified parts of the body thicker; and a method for manufacturing the plastic container.